1. Field of the Invention
The present invention relates to a ceramic electronic part such as, for example, a monolithic capacitor, a ceramic inductor, or the like, and particularly to a ceramic electronic part in which the structure of terminal electrodes formed on the outer surface of a ceramic element assembly is improved.
2. Description of the Related Art
Conventional methods of forming terminal electrodes of ceramic electronic parts include coating and baking of conductive paste, wet plating, thin film deposition such as vapor deposition, sputtering, and the like. In order to enable surface mounting of ceramic electronic parts on a printed circuit board, a metal layer having excellent solderability is frequently formed on the outermost surface of a terminal electrode.
FIG. 7 shows a monolithic capacitor 51 as an example of a conventional ceramic electronic part. Monolithic capacitor 51 comprises a ceramic sintered compact 52 made of a dielectric ceramic material. The ceramic sintered compact 52 contains internal electrodes 53a to 53f which are arranged to overlap each other with ceramic layers held therebetween. The internal electrodes 53a, 53c and 53e extend to an end surface 52a of the ceramic sintered compact 52 and the internal electrodes 53b, 53d and 53f extend to an end surface 52b. Terminal electrodes 54 and 55 are formed to cover the end surfaces 52a and 52b, respectively.
Each of the terminal electrodes 54 and 55 includes a first electrode layer 54a or 55a, a second electrode layer 54b or 55b, and a third electrode layer 54c or 55c which are laminated together. The first electrode layers 54a and 55a are formed by coating and baking a conductive paste which is mainly composed of a metal powder of Ag, Pd, Agxe2x80x94Pd, Cu, or the like, and are electrically connected to the internal electrodes 53a, 53c, and 53e or the internal electrodes 53b, 53d and 53f with excellent reliability. The third electrode layers 54c and 55c are formed by plating Sn or a Sn alloy, for improving solderability. The intermediate second electrode layers 54b and 55b are respectively provided for preventing diffusion of Ag from the first electrode layers 54a and 55a into solder during soldering, and preventing diffusion of Sn, contained in the third electrode layers 54c and 55c and the solder used in soldering, into the first electrode layers 54a and 55a and the ceramic sintered compact 52, or at least decreasing the diffusion rate. Diffusion of Sn to the interfaces between the terminal electrodes 54 and 55 and the ceramic sintered compact 52 deteriorates the adhesion of the terminal electrodes 54 and 55 to the sintered compact 52, thereby failing to obtain desired electric characteristics.
This is due to the fact that the adhesion between the terminal electrodes 54 and 55 and the ceramic sintered compact 52 depends upon oxygen bonding between the metal which forms the terminal electrodes 54 and 55, and the ceramic material, and the oxygen bonding between the electrode forming metal and the ceramic material is broken in the presence of Sn, resulting in a deterioration of the adhesion.
Therefore, the monolithic capacitor 51 includes the second electrode layers 54b and 55b to prevent the diffusion of Sn or to at least decrease the diffusion rate. Each of the second electrode layers 54b and 55b comprises a deposited film of a metal such as Ni, Ti, Cr or Mo, or an alloy thereof.
The above-described terminal electrodes having the three-layer structure are used in not only monolithic capacitors, but also various other ceramic electronic parts such as a monolithic varistor, a monolithic piezoelectric part, and the like; a single-chip ceramic capacitor, a ceramic thermistor, and the like, which are not monolithic.
While, the electrode layers 54b and 55b (comprising a deposited film of a metal such as Ti, Ni, Cr or Mo, or an alloy mainly composed of any of these metals) prevent (or slow down) the diffusion of Sn to the first electrode layers 54a and 55a, respectively, in some cases, the plating solution used to deposit the metal film enters the ceramic sintered compact 52. Therefore, the interfaces between the ceramic material, which forms the ceramic sintered compact 52 and the internal electrodes 53a-53f are reduced resulting in defects such as layer separation, cracks, etc., and failure to obtain desired electric characteristics such as electrostatic capacity. Particularly, in the formation of a single film of a metal having high affinity for oxygen, such as Ti, Cr, Mo, or the like, in a ceramic electronic part used at a relatively high voltage, i.e., in a medium-to-high-voltage region, there is oxygen depletion in the ceramic material located at the end surface 52a of the ceramic sintered compact, significantly deteriorating electric characteristics and conductivity with reduction of ceramic. The effectiveness of Ni in preventing diffusion of Sn to the first electrode layers 54a and 55a insufficient, compared to Ti, Cr, and Mo.
Accordingly, an object of the present invention is to solve the defects of the above-described conventional technology, and provide a ceramic electronic part comprising terminal electrodes capable of suppressing diffusion of Sn contained in solder into a ceramic element assembly, and causing less reduction of ceramic, thereby securely exhibiting desired electric characteristics.
The present invention provides a ceramic electronic part comprising a ceramic element assembly, and terminal electrodes formed on the outer surface of the ceramic element assembly, wherein the portions of the terminal electrodes, which adhere to the surface of the ceramic element assembly contain both a main component element which forms the primary component of said portions of the terminal electrodes, and a metal having higher affinity for oxygen than Sn.
In the terminal electrodes of the ceramic electronic part of the present invention, the metal having higher affinity for oxygen than Sn may be mixed in the terminal electrodes, or may be alloyed with the main component metal. More generally, the metal having higher affinity for oxygen than Sn can be present in various forms in the terminal electrodes.
The terminal electrodes may comprise a single electrode layer, or a lamination of a plurality of electrode layers. In the terminal electrodes comprising a lamination of a plurality of electrode layers, a layer which adheres to the ceramic element assembly contains the metal having higher affinity for oxygen than Sn.
As the metal having higher affinity for oxygen than Sn, it is preferable to select at least one metal from the group consisting of Ti, Mo, W, and V.
The metal having higher affinity for oxygen than Sn is mixed so that it is present in the faces of the terminal electrodes which adhere to the ceramic element assembly at a ratio of 0.1 to 20% by weight of the main component metal.